Induction motor, also known as asynchronous motor, is an AC motor which drives a rotor to rotate with an electromagnetic torque produced by an interaction between a rotating magnetic field formed by a stator winding and a magnetic field of induced currents in a rotor winding. FIG. 1 shows an equivalent circuit of an induction motor.
Generally, the value of the air-gap flux is kept unchanged during the adjustment of the speed of the induction motor. If the magnetic flux is very small, the iron core of the motor cannot be fully utilized and therefore cannot output a high torque. However, if the magnetic flux is very large, the iron core of the motor will be saturated, which leads to an excessive exciting current and even a failure of the motor. For this reason, frequency conversion speed control must be performed on the premise of a constant air-gap flux.
According to the motor theory, in a three-phase induction motor, the effective value of the potential for each phase of a stator is expressed as:|es|=4.44f1NskNsψm  (1-1)in which:
es represents an amplitude of an induced electromotive force of the air-gap flux (or mutual flux) in the winding for each phase of the stator;
f1 represents a current running frequency or a current frequency in the stator;
Ns represents the number of turns in series of the winding for each phase of the stator;
kNs represents a factor of the fundamental wave winding of the stator;
ψm represents an amplitude of the air-gap flux for each pole.
According to equation (1-1), the air-gap flux linkage can be kept constant as long as the follow condition is satisfied:
            e      s              f      1        =      a    ⁢                  ⁢    constant    ⁢                  ⁢    value  
Typically, es is replaced by a stator voltage since the counter electromotive force based on the air-gap flux cannot be measured directly. That is:
                          u        s                          f      1        =      a    ⁢                  ⁢    constant    ⁢                  ⁢    value  |us| represents the amplitude of the stator voltage us. This is well-known as V/F control principle. In brief, the rotate speed is changed by utilizing the frequency, and the flux linkage is kept constant by keeping |us|/f1 unchanged.
FIG. 2 illustrates a process of basic V/F control. The process comprises: setting an amplitude of the voltage according to the V/F curve after a frequency has been set, and controlling an inverter through PWM modulation to output a three-phase AC voltage so as to control the operation of a motor. FIG. 3 illustrates a specific method of basic V/F control, wherein ω*e represents an angular velocity obtained from the frequency set by a user. As discussed above, the flux linkage can be kept constant by keeping |Us|/f1 unchanged. In order to keep |us|/f1 unchanged, only the value of Ub/ωb needs to be kept unchanged (Ub represents a base value of the voltage and ωb represents a base value of the angular frequency). Then, a desired stator voltage Usqe can be obtained by multiplying ω*e by Ub/ωb. A rotation angle θ of the stator voltage vector can be obtained by integrating the above ω*e. Thereby, phase voltages UA, UB and UC of the three-phase stator can be obtained by performing coordinate transformation (including a transformation from rotating orthogonal coordinate system dq to static two-phase orthogonal coordinate system αβ and a transformation from two-phase orthogonal coordinate system αβ to three-phase ABC coordinate system) on Usqe and θ. A 6-channel PWM waveform can be output by applying the voltage space vector PWM (SVPWM) control technology to the phase voltages UA, UB, and UC. The 6-channel PWM wave controls a switch tube of a three-phase inverter to achieve the control of an induction motor.
There is no oscillation when a motor runs under ideal conditions. However, when a frequency converter controls the motor, the oscillation of the output current may occur for the following reasons.
1) In addition to a fundamental component, a PWM waveform output also includes low-order and high-order harmonics. The harmonic current will cause torque ripples. Especially in the case that the frequency is low, the oscillation iii tends to occur since the moment of inertia of the motor and the mechanical load is small but the rotate speed ripple is large.
2) A dead zone factor. Since a dead time which can affect the fundamental voltage is set in the converter to prevent the bridge arms of the inverter from shoot-through, the low-order harmonic component is enlarged, which leads to is a distortion of the current, especially when the motor is in a no-load condition. That is, the oscillation tends to occur when the motor is in a no-load condition.
3) Since the V/F control belongs to the current open-loop control, rather than the closed-loop control based on the feedback current, the corresponding adjustments cannot be made when the current fluctuates slightly. Thus, the amplitude of the current fluctuations may increase and finally cause the current oscillation, until the converter alarms.
As described above, the V/F control belongs to the open-loop control, the output of which needs to be adjusted when the current fluctuates slightly. At present, methods for suppressing an oscillating current of a motor are mainly focused on fine tuning of the voltage output and frequency output.
A specific realization process for suppressing the oscillation by fine tuning of the voltage output comprises: setting a proper cut-off frequency for a sampled reactive current isde or a sampled active current isqe; extracting, by a filter, harmonic components; and obtaining a voltage compensation based on the harmonic components. Specifically, the compensation is obtained by superposing an extracted disturbance component of the stator current onto a given stator voltage.
A specific realization process for suppressing the oscillation by fine tuning of the frequency output comprises: setting a proper cut-off frequency for a sampled reactive current isde or a sampled active current isqe; extracting, by a filter, harmonic components; and obtaining a frequency compensation of the harmonic components by multiplying the harmonic components by different values as required. Specifically, the compensation is obtained by superposing an extracted disturbance component of the stator current onto a given frequency.
In the existing technology for suppressing the oscillation by fine tuning of the frequency output, since both of the reactive current isde and the active current isqe are multiplied by the same coefficient, only the gain coefficient and the cut-off frequency of the filter are adjustable, and thereby the effect of the control method for suppressing the oscillation is limited.